1. Field of the Invention
This invention describes a method for measuring, controlling and optimizing the bed permeability of, i.e., gas flow-through, sinter mixes on a travelling grate.
2. Prior Art
The output of a sinter plant depends largely on the rate that gases will flow through the sinter mix on the travelling grate. As the sintering process takes place, the ability of the gases to flow through a given layer of sinter mix depends on the physical or chemical property of the mix or its components, and also on the pretreatment given the sinter mix. Important factors or parameters are the size consist of the mix, the water content and wetability of the components, the types of additives and the proportion of return fines. The permeability of the mix is also dependent on the porosity of the particles, the method of handling while transporting the mix to the machine and the care with which the mix is metered to the sinter machine.
The bed permeability will also depend on the operation of the ignition furnace, as too high a heat input can fuse the surface and seal the bed against air flow. The ignition furnace is the area over the first windboxes where the sinter mix is ignited by hot combustion products introduced in a hood over the bed and the hot gases are induced to flow through the bed by the suction in the windboxes under the bed.
The ignition furnace will consist of an ignition or combustion zone where hot combustion products are used to sinter the surface of the bed. The furnace can also have an additional annealing zone where preheated air is induced to flow through the sintered bed surface to assist the sintering process if desirable.
To achieve maximum sinter machine utilization, the machine speed should be regulated in relation to all factors or parameters that affect bed permeability. The impact of any of the above-mentioned factors will vary with different ore mixtures, so that factors which bear heavily on one type of ore mixture may be relatively insignificant for others.
It has, therefore, been customary to optimize only those factors which have the most influence of bed permeability in a given case to obtain the highest possible machine speed and plant output.
Sinter machine speeds have been adjusted according to flow measurements taken in the area of the actual burn-through point on the sinter machine. Any change in location of the burnthrough point will also change the location of the measurement setup, unless several measurement setpoints have been arranged in the vicinity of the burn-through point. The latter arrangement will permit recognition of changes in burn through, and by adjusting the machine speed, the location of the burn-through point can be changed with respect to the gas flow-through, or bed permeability of, the sinter mix. This method, however, will have no bearing on the actual combustion process, the heat input, and other factors which might influence the ignition zone proper. The cost for one or more adjustable setups to measure gas flow is considerable because separate systems to measure gas flow will be required. (See German Patent No. 1,961,297.)
The impact of humidity on quality and output of sinter plants has resulted in a process to control such moisture additions (See German Patent No. 1,811,281), in order to adjust the gas flow rate of green pellets or sinter mix by measuring such gas flow rates in a separate bin or on the travelling grate before the mix reaches the ignition furnace.
There is, however, no process available by which gas flow in the actual furnace can be controlled for the purpose of output maximization. Yet, this range is the most important part of the entire process because the mode of combustion in this area will control the quality of the sinter to a considerable extent. The machine speed for the combustion process taking place in this zone will determine the outcome of the entire process. Measurements for gas flow by conventional methods appear to be very difficult in a hot gas zone, therefore, it is customary to determine gas flow in front of, or behind, this area with respective instrumentation.
Values that have been established with prior art methods, will yield only relative results, as either derived from one of the given factors or process parameters such as mix moisture content or location of the burn-through point. These values have no bearing on the heat input within the furnace, nor will they accommodate for a change in the rate of gas flow through the sinter mix in the furnace resulting from compression of the mix by the vacuum incurred through suction. Gas flow in the combustion zone will nevertheless remain the determining factor in the reaction of various ore mixtures, their pretreatment, as well as the quality of the sinter product.